Pham Van Hai, 31, was found guilty of "illegally posting or using information on computer networks and telecommunication networks," an offense that carries a jail term of up to seven years.
Last year, after breaking up with his girlfriend, Hai took over her Facebook account. On April 10 this year in the account he posted that Thai Nguyen had had a Covid-19 death but the government had concealed it.
When many of his friends asked about it, he claimed this was "true" and the dead person was a 27-year-old man.
He received hundreds of likes and shares, and sparked fear among local people.
Since the pandemic broke out in late January, authorities have slapped fines of up to 15 million ($427- 641) on hundreds of individuals for posting fake news.
At a meeting earlier this month Prime Minister Nguyen Xuan Phuc ordered stricter penalties for those posting fake news on Covid-19.
Now authorities are resorting to criminal charges with attendant jail terms of up to seven years.
Two men in the central provinces of Nghe An and Lam Dong also face criminal charges for making false claims on social media about Covid-19 deaths in Vietnam and a government cover-up.
Vietnam has recorded 268 Covid-19 cases, 44 of them active, and there have been no deaths.